feat(fuse): add client-side metrics via MessagePack/ZeroMQ

Ahmad Tarrafrequested to merge
feat/fuse-client-metrics into master
  • Add GKFS_ENABLE_CLIENT_METRICS to fuse_client and gkfs_user_lib targets
    • Instrument read/write handlers with add_event() (skip on error to avoid heap corruption)
    • Call init_metrics() on startup and flush on shutdown
    • Same env vars as intercept client: LIBGKFS_ENABLE_METRICS, LIBGKFS_METRICS_IP_PORT, etc.
    • Closes #387

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